1. Field of the Invention
The present invention relates to a method of tightening fasteners such as bolts and nuts and more particularly it relates to prevention of variation in axial force during tightening fasteners using a nut runner.
2. Prior Art
In automobile engine assembling operation, automatic tightening of bolts and nuts by a nut runner is performed. As for means for bolting engine components, such as a cylinder head and a cylinder block, with a predetermined tightening torque, use is made of a first method (torque method) comprising the steps of presetting the final tightening torque for a nut runner, detecting the actual torque when a bolt is tightened, and terminating the bolt tightening operation using the nut runner when the final tightening torque equals the actual torque, and a second method (torque-angle method) comprising the steps of presetting a snug torque for a nut runner which is about one third as large as the final tightening torque and stopping the rotation after the snug point has been passed.
The torque method described above is intended to impart a predetermined tightening strength to the fastener by finding and controlling the tightening force on the fastener. However, the tightening strength is, in fact, determined by the size of the transmitted axial force (hereinafter referred to as the axial force). This axial force changes due to the friction coefficient for the screw surface and seat surface, so that in most cases the tightening torque and axial force do not have the same value. Therefore, even if the tightening torque is controlled to be constant, there sometimes occurs a dispersion in axial force which is as much as 100%, as shown in FIG. 5. Stated more quantitatively, between the tightening torque T and the axial force F, the following relations hold: ##EQU1## (where D is the diameter of the fastener and K is a torque coefficient).
The friction coefficient .mu., which is a main element of the torque coefficient K not only varies greatly due to the characteristics of the seat surface for the fastener and the thread surface but also is influenced by the rotative speed at which the fastener is being tightened. That is, as shown in FIG. 6, the friction coefficient is high in the static friction region where the rotative speed of the nut runner is low (normally, 3 RPM or less) and is low in the dynamic friction region where the rotative speed of the nut runner is high. From this point of view, it is seen that in order to reduce the dispersion in axial force F, it is preferable to tighten fasteners in the dynamic region where friction coefficient .mu. exercises less influence and with a constant speed by using a nut runner. In the conventional torque method, however, the final tightening torque T is set in disregard of the relationship between axial force F and friction coefficient .mu., with the result that in the final period of bolt tightening operation when the snug point is passed, the rpm gradually decreases until the final tightening torque is reached. More particularly, in the conventional system, since the drive motor for the nut runner is driven at a given voltage without having its speed controlled, the rpm of the nut runner gradually decreases as the actual tightening torque increases. This indicates the series characteristic of DC motors; series motors and commutator motors having their speed uncontrolled exhibit such characteristics. Nut runners having their rpm uncontrolled are attended with a change in rpm, such a change in rpm influencing the friction coefficient to cause a dispersion in axial force. As shown in FIG. 6, when the rpm of the nut runner decreases to a point in the vicinity of the boundary between the dynamic and static friction regions, the change in friction coefficient .mu. suddenly increases, producing a large dispersion in axial force F.
Further, in the torque-angle method, since the snug point is set by torque, the same problems as described above arise.